fix(ng:options): compile null/blank option tag

[TEHEK]

The latest version was throwing some cryptic
cannot access childNodes property of undefined
when using template in select's blank option.

This one required going really deep down to compiler. Turns out that when Template instance is created, it saves paths to (indexes of) child nodes which are to be compiled, then executes linker functions on given element and then proceeds to it's childNodes.

When had ng:options, it removed all from causing Template go try linking with unexistent element's childNode.

• added guard check for compiler
• refactored src/widget/select.js to compile blank option (I tried several options, but this one seems the most natural)
• added some tests

Fixes #562

[IgorMinar]

I cleaned up the commits and landed the changes. Thanks!

[angular.js] IgorMinar pushed 2 new commits to master: http://git.io/Wiy4aA

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